1. Field of the Invention
The present invention relates to an element having at least one layer that changes its electromagnetic absorption characteristics on the application of heat. This layer is useful, for example, as a filter layer or an antihalation layer in a photographic element, such as a photothermographic element. The element is also useful as a thermographic recording element. The element of the present invention has at least one layer containing a benzopinacol and a bleachable dye or dye precursor.
2. Discussion of the State of the Art
A large number of photographic elements are known which can be processed by immersion in various processing solutions. It is also known to provide various filter layers including filter overcoat layers, filter interlayers and antihalation layers, to improve photographic reproduction. The filtering capability of these layers is generally destroyed during the processing of these elements by one of the processing solutions so that the processed element is entirely transparent in the visible region.
Imaging elements are also known which can be processed, after imagewise exposure, simply by heating the element. These elements include known heat developable photographic elements, also known as photothermographic elements. Like conventional elements, it is desirable to provide heat processable elements with various filter layers. In most cases, these filter layers must be rendered substantially transparent by the heat processing. Typical heat developable photographic elements are described, for example, in U.S. Pat. No. 3,220,846 of Sagura and Tinker, issued Nov. 30, 1965; U.S. Pat. No. 3,301,678 of Humphlett, Johnson and Haist, issued Jan. 31, 1967; British Pat. No. 1,161,777 published Aug. 20, 1969; U.S. Pat. No. 3,392,020 of Yutzy and Yackel, issued July 9, 1968; British Pat. No. 930,572 published July 3, 1963; U.S. Pat. No. 3,152,904 of Sorenson et al, issued Oct. 13, 1964; U.S. Pat. No. 3,457,075 of Morgan et al, issued July 22, 1969; and U.S. Pat. No. 3,801,321 of Evans and McLaen, issued Apr. 2, 1974.
The antihalation layer of a photographic element prevents light that has passed through the radiation sensitive layer from reflecting off of the support and back into the light sensitive layer. If not prevented, this reflected light could reduce the sharpness of the image. Antihalation layers, and other filter layers, have been suggested for use with heat developable photographic elements. One such antihalation layer is described in U.S. Pat. No. 3,745,009 to Jenkins, Heseltine and Mee. This patent describes a class of dyes which change from color to colorless on exposure to heat or light. No separate activating component is used. These dyes provide antihalation protection; however, these dyes generally require higher temperatures than desired before they decompose. In U.S. Pat. No. 3,769,019 of Wiese et al an antihalation layer is described wherein the acid component of a dye is neutralized by a heat generated base. In another embodiment described in this patent, decoloration of the dye is accomplished by removal of an acid portion by heat. Unfortunately, decoloration of the dye is not always as permanent as desired. U.S. Pat. No. 3,821,001 of Weber describes another antihalation layer containing an alkali bleachable vanadium complex of 8-hydroxyquinoline and a heat labile alkali precursor. In many cases higher temperatures than desired are required to provide the degree of decoloration desired. Photobleachable antihalation dyes are also known, as described, for example, in U.S. Pat. No. 3,984,248 of Sturmer, issued Oct. 5, 1976. It is often desired that the antihalation layer not be photosensitive.
A continuing need has existed for improved antihalation layers and other filter layers, that (a) can be decolorized simply by the application of heat but which are stable during storage at ambient conditions and (b) provide improved permanence in maintaining the desired degree of transparency of the layers after processing.
Typically, layers which can be used as antihalation layers, or other filter layers in a photothermographic element, can also be useful alone on a support to provide a thermographic element. In these embodiments, any color change can be useful to form an image. For example, a substantially colorless dye precursor can be imagewise thermally bleached to form a positive colored image. Of course, a colored dye can be imagewise thermally bleached to a colorless state thereby forming a negative image. A variety of thermographic materials are known that use this concept. For example, U.S. Pat. No. 3,852,093 of O'Leary describes a thermographic element having a layer containing an imine dye and a mild reducing agent. Brief imagewise heating of the element causes migration of the reactants which results in decoloration in the imagewise heated area. All of these thermographic elements are useful for making reflex copies, writing with heated styluses and laser writing. Examples of these embodiments are described in U.S. Pat. No. 3,745,009 of Jenkins et al.
All of the heat bleachable elements described have at least one of the disadvantages: (1) frequently high temperature is required to bleach the dye, (2) in some cases even when the dye is bleached it has a tendency to recombine forming undesired stain or discoloration on prolonged post process keeping, and (3) choice of suitable dyes is greatly limited.